The advanced computer network like the Internet accelerates digitalization of information and allows a large number of users to readily access required information. Such advanced environment, on the other hand, causes digital contents including digital information with the copyright to be readily duplicated as unauthorized copies without permission of copyright holders. Infringement of the copyright due to the illegal copies is becoming a significant problem. With a view to preventing the infringement of the copyright with regard to color images, which are the main part of digital information in the digital contents, the digital watermarking technique draws the vigorous attention. The digital watermarking technique embeds the copyright information in data of a master color image in a secret and unnoticeable manner to any third person and extracts the copyright information from the embedded master color image according to the requirements.
One known technique of embedding a digital watermark in two-dimensional data like image data causes the image data to be subjected to orthogonal transform, such as discrete cosine transform, and changes the resulting coefficients based on a predetermined algorithm. In this technique, the master image is closed to the public. The procedure processes both the master image and a potential illegal copied image by the orthogonal transform and compares their coefficients for identification of the copied image.
This prior art technique requires the master image, which is closed to the public, for extraction of the copyright information and causes another significant issue, management of the master image. The increase in number of master images makes the management more difficult. It is accordingly desirable to embed a digital watermark in a specific form that allows direct extraction from the image.
The digital watermark is also required to have sufficient resistance against the partial clipping of images and overwriting attacks. There are a diversity of attacks like Laplacian attacks against the digital watermarking that utilizes the orthogonal transform. No practically effective techniques have been proposed so far to solve the problems like the attacks and the partial clipping of images.